Remove duplicate section in add-repository-instructions.md

[mattbrannon]

Why:

Closes: #43525

What's being changed (if available, include any code snippets, screenshots, or gifs):

Removes the duplicate line found at https://docs.github.com/en/copilot/how-tos/configure-custom-instructions/add-repository-instructions#creating-repository-wide-custom-instructions

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

[github-actions]

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Source	Review	Production	What Changed
copilot/how-tos/configure-custom-instructions/add-repository-instructions.md	fpt ghec	fpt ghec

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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[Copilot]

Pull request overview

Removes a duplicated sentence in the “Creating repository-wide custom instructions” section of the Copilot docs article, addressing the duplication reported in issue #43525.

Changes:

• Deleted a repeated paragraph under “Creating repository-wide custom instructions” to eliminate duplicate content.